Gross anatomic, histologic, histochemical and electron microscopic studies were made to determine the modes failure of glutaraldehyde- treated and antibiotic-preserved mitral valve allografts implanted in sheep for a mean of 20 weeks. Te glutaraldehyde-treated allograft failed because of severe, progressive calcification. The antibiotic- preserved allografts failed because of deterioration of connective tissue and infiltration of inflammatory cells.